1. Field of the Invention
The invention relates to mutant human P-glycoprotein polypeptides that confer increased resistance to certain chemotherapeutic drugs compared to wild-type P-glycoprotein or P-glycoprotein having a glycine to valine substitution at position 185, and nucleic acid molecules encoding the same. The invention also relates to antibodies that specifically bind mutant human P-glycoproteins. The invention further relates to methods for diagnosis and treatment of conditions associated with P-glycoprotein-mediated multidrug resistance.
2. Background of the Invention
Multidrug resistance (MDR) is a phenomenon in which a cell becomes resistant to a large group of structurally diverse chemotherapeutic drugs that act at different intracellular targets. One mechanism of MDR1 in clinical cancer is active drug efflux mediated by a cellular protein termed P-glycoprotein. P-glycoprotein is an integral membrane protein having six transmembrane domains and consisting of 1280 amino acids (SEQ ID NO: 2), which functions as a broad specificity efflux pump. The protein is composed of two structurally similar portions, of approximately 600 amino acids each, which are separated by a linker region. Human P-glycoprotein is encoded by the MDR1 gene (Chen et al., 1986, Cell 47:381-89), the wild-type cDNA sequence of which is identified herein as SEQ ID NO: 1.
A number of mutant P-glycoproteins, possessing an altered ability to transport different drugs, have been isolated. Most of these mutants show decreased drug efflux, while only a small number of mutants exhibit increased drug transport. Examples of the latter group are the MDR1-G185V (Choi et al., 1988, Cell 53:519-29; Safa et al., 1990, Proc. Natl. Acad. Sci. U.S.A. 87:7225-29) and MDR1-H61F mutants (Taguchi et al., 1997, Biochemistry 36:8883-89). Mutant P-glycoproteins showing increased resistance to certain chemotherapeutic drugs also show decreased resistance to other drugs.
Efforts to treat clinical cancer with cytotoxic drugs are often hampered by sensitivity of non-cancerous proliferating cells, such as hematopoietic cells, to the cytotoxic effects of these drugs. One approach to overcoming this shortcoming is to genetically engineer hematopoietic cells, particularly precursor cells such as stem cells, to express elevated levels of P-glycoprotein in order to make these cells more resistant to the cytotoxic effects of chemotherapeutic drugs, enabling higher doses of such drugs to be administered for more effective anticancer treatment. These efforts would be even more effective if the non-cancerous cells could be made preferentially resistant to a chemotherapeutic drug that was particularly appropriate for treatment of a specific cancer.
Additionally, expression of different mutant P-glycoproteins in clinical cancer can make inappropriate a specific choice of chemotherapeutic drug in an individual patient having a neoplastic disease conventionally treated with said specific drug. Identification of specific mutants in clinical cancer would provide clinicians with information enabling rational, individualized drug treatment choices in anticancer treatment.
Thus, there remains a need in the art for mutant P-glycoproteins that confer increased resistance to certain chemotherapeutic drugs relative to wild-type P-glycoprotein or P-glycoprotein having a glycine to valine substitution at position 185. The development of such mutants would have wide application in the treatment and diagnosis of cancer.